Internal combustion engines may include an exhaust gas recirculation (EGR) system to recirculate a controlled portion of exhaust as generated by the engine into an intake manifold of the engine. The recirculation of exhaust gas provides various benefits including a reduction in exhaust NOx emissions and improved fuel economy. However, the dilution provided by the EGR results in a change in the inferred octane of the fuel injected into the engine. Ambient humidity also results in a dilution effect by affecting the charge cooling ability of the intake aircharge, in addition to displacing the oxygen in the air and hence reducing its concentration. Specifically, humidity reduces the knock tendency the engine by lowering the temperature of the end gases in the combustion chamber.
Various engine control approaches have been developed to account for the change in inferred octane of a fuel based on the presence of EGR and/or humidity. One example approach is shown by Cullen et al. in U.S. Pat. No. 6,062,204. Therein, spark timing is determined as a function of engine speed-load and then corrected with a factor that is based on the dilution effect of EGR and/or humidity on borderline spark.
However, the inventors herein have recognized a potential issue with such an approach. The octane effect of EGR and humidity varies with engine speed-load conditions. The inferred octane strategy of Cullen determines the octane of the fuel based on the fuel type and independent of the engine speed-load. As a result, the octane effect of EGR and humidity may cause an error in the calculation of the inferred octane. For example, in the presence of EGR or high humidity, the inferred octane may be over-estimated (e.g., the apparent octane effect of the EGR/dilution may be interpreted as a real fuel octane addition by the inferred octane algorithm). Spark adjustments based on the incorrect inferred octane may result in degraded fuel economy and as well as unexpected knock. Specifically, under high humidity conditions and in the absence of humidity compensation, an engine controller may not have accurate knowledge of the ambient humidity level and hence may perform a knock adaptation that over-protects against knock. As such, this may affect overall engine performance.
In one example, the issues described above may be addressed by a method for controlling an engine comprising: selecting an initial spark timing based on engine operating parameters including estimated fuel octane; learning a spark timing adjustment based on feedback indication of knock; correcting the spark timing adjustment based on the engine operating parameters; updating the initial spark timing based on the corrected spark timing adjustment; and updating the estimated fuel octane based on the updated spark timing.
The technical effect of adjusting a spark control adaptation with a modification based on ambient humidity is that a more reliable inferred fuel octane estimate may be achieved, and the speed-load effect of humidity on fuel octane can be reduced. By improving the accuracy of a fuel octane estimate, borderline spark settings may be better set, and spark timing may be provided without significant loss in torque. Hence, the convergence of the octane number estimation is faster, more robust, and independent of speed load fluctuations.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.